Southern blight of vegetables and herbaceous plants

Southern blight is one common name for a disease caused by the fungus Sclerotium rolfsii. Other common names include crown rot and white mold. At one time Southern blight was thought to be a disease problem only in warm climates. In recent years, however, incidents of the disease have been reported from Illinois, Iowa, and Michigan, as well as here in Minnesota. It was thought that the overwintering structures of this fungus were not cold hardy, but they have now been shown to survive winters when under cover of snow and/or mulch. Unfortunately, Southern blight has a host range of approximately 200 different genera including ornamental plants and field crops. Some of the more common hosts include daylily, astilbe, hostas, peony, phlox, ajuga, delphinium and potato.

Symptoms and signs

Fig 1. Mycelium and sclerotia growing on infected hosta stem

Diagnosis of Southern blight can be made by looking for the signs of the fungal growth [Fig. 1]. S. rolfsii causes symptoms that can be confused with other crown or stem rots. Usually the first symptom noticed is the yellowing and wilting of lower leaves. Leaves will also die back from the tips down and very succulent stems will fall over. The fungus typically attacks the plant just under or at the soil line. Sometimes a dark brown lesion can be seen on the stem before other symptoms are visible [Fig. 2]. Hosta growers report a brown rot at the base of the petioles of wilted leaves. The fungus produces a large amount of cottony white, thread-like material called mycelium, which can grow up the stems of plants and also spread out across the soil to infect other plants. A key diagnostic feature is the overwintering structure, called sclerotia. The sclerotia of Sclerotium rolfsii are small, round, and typically a brown or tan color when mature [Fig. 3]. Another common name, the mustard seed fungus, refers to the appearance of the sclerotia. These sclerotia will be visible on the soil around an infected plant as well as on infected plant tissue. Southern blight can be confused with white mold (Sclerotinia sclerotiorum), another fungus with a broad host range that produces white, cottony mycelia. Sclerotia produced by white mold are brown-black, resemble rodent droppings, and are usually found on the inner stem of infected plants.

Management

Fig 2. Daylily showing early signs of infection with S. rolfsii.

Control of Southern blight is a challenge. Research is being done on resistant varieties, but at this point, information on less susceptible plants is mostly anecdotal. It has been reported that ornamental grasses and woody plants, such as ornamental shrubs, are not susceptible to the disease. With Southern blight, prevention is the best management strategy. Inspect all new plants and their associated soil carefully for the symptoms and signs noted above. If you have any doubts, consider putting a new plant in a quarantinearea of your garden, where, if it is diseased, the disease can not spread to other plants.

Fig 3. Sclerotia on astilbe

If S. rolfsii becomes established in your garden, there are some important cultural controls you can implement to reduce the spread of this disease. When dealing with Southern blight, sanitation is particularly important. Sclerotia can be transported around your garden with infected soil. For this reason, carefully clean soil off your tools and even your shoes. Transplanting infested plants is another way the fungus is spread around a garden. Because the fungus can overwinter in mulch, it is helpful to remove mulch from the crowns of the plants. Soil solarization, a process that heats the soil to levels sufficient to kill many fungi, is a control measure used in the south. Northern gardeners should be aware that the soil in our climate will not reach temperatures high enough to kill the fungus. The fungicides for Sclerotium rolfsii control are not available for homeowners to purchase at retail centers. If you have determined that your plants have Sclerotium rolfsii, and other control measures are unsuccessful, you may wish to contact a commercial applicator to discuss the use of chemical controls.